1. Field of the Invention
The present invention relates to a switching power supply unit which regulates an output voltage using a digital control circuit.
2. Description of the Related Art
In a related art switching power supply unit, an output voltage is regulated by, for example, pulse width modulation (PWM) control or pulse frequency modulation (PFM) control with a highly intelligent digital control circuit using, for example, a digital processor. Recently, various switching power supply units provided with high speed digital control for the reduction of a transient fluctuation in an output voltage in response to a sudden change in an external environment of the power supply unit have been proposed.
For example, as disclosed in Japanese Patent Application Laid-open No. 2008-125286, an exemplary related art switching power supply unit includes: a deviation calculation means which calculates a deviation from a difference between a sampled output voltage and a reference voltage; a deviation estimate calculation means which estimates a deviation in the next cycle; and a controlled variable computing means which calculates a controlled variable for the control of a pulse width of a switching element. A coefficient α is included in an estimate calculation formula (linear model) defined in the deviation estimate calculation means and thus the controlled variable can be adjusted by changing the coefficient α.
As disclosed in Japanese Patent Application Laid-open No. 2005-184964, another exemplary related art power unit performs calculations based on a value of a sampled input voltage and an output voltage, obtains the ON duration and the OFF duration of a main switching element independently and controls the main switching element to turn on or off using the obtained ON or OFF duration. In the calculations, each deviation is obtained from the sampled input voltage and a difference between the output voltage and the reference voltage, and calculates the ON duration and the OFF duration in which the output voltage is equal to the reference voltage.
As disclosed in Japanese Patent Application Laid-open No. 2008-99539, a further exemplary related art switching power supply unit includes: a determination means which determines whether a difference between a sampled output voltage and a reference voltage (i.e., a target voltage) is in a predetermined range; a controlled variable calculation means which calculates a controlled variable of an output voltage based on, for example, the difference and an average value of the output voltage; and a control signal output means which controls the main switching element to be turned on or off based on the calculated controlled variable. The controlled variable calculation means accelerates the control by reducing the number of sampling or increasing the controlled variable by changing a coefficient of a controlled variable calculation formula when the fluctuation in the output voltage exceeds a reference value.
Here, exemplary situations of rapid fluctuations in an external environment of the switching power supply unit may include a rapid fluctuation in the input voltage during operation or a sudden increase or decrease in a load current in response to a change in a state of the load. An operation of the switching power supply unit when an input voltage is placed in a non-operating switching power supply unit is also a sudden change in the external environment.
The switching power supply unit used in, for example, information and communication equipment or and a server, should have high speed response characteristics of the output voltage control in response to a sudden change in the load because a sudden change in the load current from about zero to a rated load may occur frequently. In a system in which voltage is supplied to many board substrates from one input power line, it is assumed that, when another board substrate is hot swap for maintenance, a loading state of the power source line of the system changes and the input voltage of the switching power supply unit which operates in response to the power supply from the power source line changes suddenly. Accordingly, high speed response characteristics of the output voltage control in response to the rapid fluctuation in the input voltage are required for such a system. If transient overshoot or vibration occur when an input voltage is placed or in a process in which an output voltage increases from zero to reach a predetermined output voltage, there is a possibility that the electronic equipment connected as a load may malfunction or break down. Accordingly, in various applications, it is required that an output voltage of a switching power supply unit increases monotonically.
In the above-described switching power supply unit disclosed in Japanese Patent Application Laid-open No. 2008-125286, since forecasting control (i.e., feed-forward control) is performed based on a sampled output voltage, a response to a sudden change in the load is rapid as compared with normal feedback control. However, the input voltage is not considered during determination of the controlled variable. Accordingly, the fluctuation in the output voltage in response to the rapid fluctuation in the input voltage cannot be reduced significantly. Further, prevention of transient overshoot or vibration when an input voltage is placed is not considered.
In the power unit disclosed in Japanese Patent Application Laid-open No. 2005-184964, the feed-forward control is performed based on the sampled input voltage and thus the change in the controlled variable is gradual and relatively slow. Thus, although the time until the fluctuation in the output voltage in response to a sudden change in the load is relatively short, the amount of the fluctuation is not significantly small. Further, prevention of transient overshoot or vibration when a voltage is input is not considered.
In the switching power supply unit disclosed in Japanese Patent Application Laid-open No. 2008-99539, it may be possible that a fluctuation in an output voltage in response to a rapid fluctuation in an input voltage or a sudden change in a load can be reduced to some extent by accelerating and increasing a gain of the response of the feedback-control of the output voltage. However, prevention of transient overshoot or vibration when a voltage is input is not considered.